The present invention generally relates to a system for coupling a DC power source to an AC power source, and specifically, to a self-commutated power converter which can continue to operate normally even when an AC power fault (power suspension) caused during an AC coupling of the system, occurs.
Conventionally, when AC power sources of different systems are to be operated in parallel, the voltage differences among these power sources are controlled according to the reactive power differences of the power sources, and the phase differences among them are controlled according to their active power differences. However, when a DC/AC power converter (known as an inverter) is coupled to a power system, the power system itself cannot be the subject of the control. For this reason, a self-commutated power converter (hereinafter simply referred to as a power converter) is generally adapted to control the active or reactive power.
FIG. 1 shows the above-described conventional power converter. In the figure, the DC output from DC power source 1 is converted to AC via power converter 2. The voltage of the converted AC power is changed to a prescribed value by power transformer 3. The AC power delivered from transformer 3 is coupled to AC power system 5 via circuit breaker 4.
In control circuit 100 of power converter 2, output Vref from voltage reference 21 is combined, at mixer 21A, with detection voltage 12a representing the voltage at coupling point CP. Error signal 21a, obtained by this mixing, is applied to error amplifier 22. Output 22a from amplifier 22 is one control input for voltage controller 23.
Similarly, output Paref from active power reference 31 is combined, at mixer 31A, with signal 32a output from active power detector 32. Error signal 31a, obtained by this mixing, is applied to error amplifier 33. Output signal 33a from amplifier 33 is supplied to phase set input "a" of phase-locked loop (PLL) 34.
The frequency of output signal 34a from PLL 34 is frequency-divided by frequency divider 35. Frequency-divided output 35a from divider 35 is then supplied, as phase feedback signal "b", to PLL 34. PLL 34 also receives, as phase reference signal "c", detection voltage 13a of power system 5. Output signal 34a from PLL 34 is supplied, as the other control input, to voltage controller 23.
Phase reference signal "c" of PLL 34 represents the voltage phase (13a) of power system 5. Accordingly, the output frequency of PLL 34 matches that of power system 5, and the voltage phase of power converter 2 is synchronized with that of power system 5.
Sync detector 42 includes a comparator circuit for comparing the phase of coupling point-detection voltage 12a with that of detection voltage 13a of power system 5. When the phase difference between voltages 12a and 13a falls within a predetermined range, detector 42 generates sync-detection signal 42a. Signal 42a is one of the closure-enabling conditions for circuit breaker 4 and is supplied to breaker controller 41.
Voltage detector 43 includes a comparator circuit for detecting, in accordance with a given reference threshold level, the value of voltage 13a of power system 5. Detector 43 supplies voltage detection signal 43a to breaker controller 41 when power system detection voltage 13a falls within the range defined by said given reference threshold level. Only when signals 42a and 43a are generated, does controller 41 output operation signal 41a for closing circuit breaker 4, and operation signal 41b for opening switch 36. Switch 36 is provided for short-circuiting the output of amplifier 33 with the input thereof when it is closed.
When circuit breaker 4 is opened and switch 36 is closed, coupling point-detection voltage 12a is automatically controlled to be equal to output Vref from voltage reference 21, so that a constant voltage corresponding to Vref is applied to load 6, provided that the power consumption of load 6 is smaller than the power capacity of converter 2. Besides, since switch 36 short-circuits the input and output of error amplifier 33, the error (33a) of active power control is substantially zero. Consequently, the active power control circuit, which controls the voltage phase of power converter 2 with respect to that of power system 5 according to the error (33a), is disenabled.
When circuit breaker 4 is closed and switch 36 is opened, the output of amplifier 33 is released from the short-circuiting with its input. Then, the voltage phase of converter 2 is automatically controlled, so that the active power of converter 2 matches output Paref from active power reference 31.
In the system of FIG. 1, when the total power of load 6 and a load (not shown) connected to power system 5 exceeds the output capacity of power converter 2, and if power system 5 is subjected to a power fault during the effective AC coupling of converter 2 with system 5, a protector (or safety device; not shown) of system 5 operates, so that controller 41 opens circuit breaker 4.
When the above total power of the loads is less than the output capacity of converter 2, even if a power fault occurs, converter 2 still supplies a certain amount of power to system 5. Then, voltage detector 43 cannot detect the power fault even if it actually occurs, and converter 2 continues its operation as if normal AC coupling with system 5 has been established.
In this case, however, converter 2 supplies power consumed only by load 6 and by the load connected to system 5. For this reason, the difference between active power reference output Paref and active power detection signal 32a becomes far greater than that obtained in a normal condition. This large difference (31a) causes output signal 33a of error amplifier 33 to be saturated, thereby disenabling the control for power converter 2. Then, because of the delay in the response of PLL 34 or the like, even if the power fault of system 5 is removed, converter 2 cannot operate in synchronism with system 5 immediately afterwards. Because of this, a large phase difference between the output voltage of converter 2 and the voltage of system 5 is temporarily generated.
Further, if an open-circuit fault occurs in active power detector 32, so that the closed feedback loop of the control circuit is cut off, output signal 33a of error amplifier 33 is saturated. Then, the active power system control is disenabled, and a large phase difference, exceeding a normal value, is generated between the output voltage of converter 2 and the voltage of system 5. That is, due to a power fault of power system 5 or a malfunction in the control of the active power control circuit, rapid exchanges of active power occur between converter 2 and system 5, and converter 2 can no longer continue its normal power-conversion operation. This is the disadvantage of the power converter of FIG. 1.
The above disadvantage can also be present in the case where control circuit 100 is provided with a reactive power control circuit, and the output of an error amplifier in the reactive power control circuit is saturated.